The present invention generally relates to semiconductor processing and, more particularly, to a system and method for measuring a sample using a scanning electron microscope and calibration thereof.
In the semiconductor industry there is a continuing trend toward higher device densities. To achieve these high densities there have been, and continue to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such a high device packing density, smaller features sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as the comers and edges of various features.
The requirement of small features with close spacing between adjacent features requires high resolution photo lithographic processes as well as high resolution inspection and measurement instruments. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which, for example, a silicon wafer is coated uniformly with a radiation-sensitive film (e.g., a photoresist), and an exposing source (such as ultraviolet light, x-rays, or an electron beam) illuminates selected areas of the film surface through an intervening master template (e.g., a mask or reticle) to generate a particular pattern. The exposed pattern on the photoresist film is then developed with a solvent called a developer which makes the exposed pattern either soluble or insoluble depending on the type of photoresist (i.e., positive or negative resist). The soluble portions of the resist are then removed, thus leaving a photoresist mask corresponding to the desired pattern on the silicon wafer for further processing.
In order to control quality in the design and manufacture of high density semiconductor devices, it is necessary to measure critical dimensions (CDs) associated therewith. Semiconductor device features having CDs of interest include, for example, the width of a patterned line, the distance between two lines or devices, and the size of a contact. CDs related to these and other features may be monitored during production and development in order to maintain proper device performance. As device density increases and device sizes decrease, the ability to carry out quick, inexpensive, reliable, accurate, high-resolution, non-destructive measurements of CDs in the semiconductor industry is crucial. The ability to accurately measure particular features of a semiconductor workpiece allows for adjustment of manufacturing processes and design modifications in order to produce better products, reduce defects, etc.
CDs are usually measured during or after lithography. Various operations performed during the lithography process may affect the critical dimensions of a semiconductor device. For example, variations in the thickness of the applied photoresist, lamp intensity during the exposure process, and developer concentration all result in variations of semiconductor line widths. In addition, line width variations may occur whenever a line is in the vicinity of a step (a sudden increase in topography). Such topography-related line width variations may be caused by various factors including differences in the energy transferred to the photoresist at different photoresist thicknesses, light scattering at the edges of the steps, and standing wave effects. Since these factors can greatly affect CDs, fast and reliable monitoring of semiconductor device features is important in order to guarantee acceptable device performance.
Different technologies are currently available to measure CDs associated with semiconductor devices. These include: optical microscopy, stylus profilometry, atomic force microscopy, scanning tunneling microscopy, and scanning electron microscopy. Scanning electron microscopes (SEMs) are commonly used for inspection and metrology in semiconductor manufacturing. The short wavelengths of scanning electron microscopes have several advantages over conventionally used optical microscopes. For example, scanning electron microscopes may achieve resolutions from about 100 to 200 Angstroms, while the resolution of optical microscopes is typically about 2,500 Angstroms. In addition, scanning electron microscopes provide depths of field several orders of magnitude greater than optical microscopes.
In a typical SEM wafer inspection system, a focused electron beam is scanned from point to point on a specimen surface in a rectangular raster pattern. Accelerating voltage, beam current and spot diameter may be optimized according to specific applications and specimen compositions. As the scanning electron beam contacts the surface of a specimen, backscattered and/or secondary electrons are emitted from the specimen surface. Semiconductor inspection, analysis and metrology is performed by detecting these secondary electrons. A point by point visual representation of the specimen may be obtained on a CRT screen or other display device as the electron beam controllably scans a specimen.
Scanning electron microscopes (SEMs) operate by creating a beam of electrons accelerated to energies up to several thousand electron volts. The electron beam is focused to a small diameter and scanned across a CD or feature of interest in the scanned specimen. When the electron beam strikes the surface of the specimen, low energy secondary electrons are emitted. The yield of secondary electrons depends on various factors including the work function of the material, the topography of the sample, the curvature of the surface, and the like. These secondary electrons can be employed to distinguish between different materials on a specimen surface since different materials may have significantly different work functions.
Topographic features also affect the yield of secondary electrons. Consequently, changes in height along a specimen surface may be measured using an SEM. Electron current resulting from the surface-emitted secondary electrons is detected and used to control the intensity of pixels on a monitor or other display device connected to the SEM. An image of the specimen may be created by synchronously scanning the electron beam and the display device.
Although SEMs can achieve resolution in the range of angstroms, calibration is difficult. For example, the magnification of an SEM may be calibrated by placing a sample of known dimensions, such as a chip or wafer having a conductor line of known width, in the instrument and measuring the dimension of the sample. The magnification of the SEM is determined by dividing the SEM measurement of the image of the sample by the known dimension of the sample. The magnification calibration information may then be used to construct a calibration curve, or the SEM""s magnification controls may be trimmed accordingly.
Calibration according to these prior methods requires samples of known dimensions. The actual dimensions of a sample, however, may not be precisely known, or may change. In particular, repeated usage of a single reference sample as a calibration standard results in degradation of the reference sample. Charge buildup on a reference sample caused by repeated measurement in a SEM affects the secondary electron emission. Contaminant deposition or buildup also has deleterious effects on measurement of a calibration standard reference sample over time. Conventional SEM calibration methods and systems do not account for the errors in estimating the actual size of a reference feature, and also fail to account for degradation in reference features.
The measurement of a calibration standard reference sample typically involves determining where an edge of the sample is. At the sub-micron range, an edge of a sample may be a complex waveform, as opposed to a flat line. Therefore, in measuring the sample there is uncertainty as to edge location. Where the calibration involves determining the length of a sample, two edges must be located, and thus the edge determination uncertainty increases. Further, sample dimensions may vary as a function of temperature, repeated measurements and electron beam charging causing contamination. The SEM electron beam may thus cause expansion of a reference sample after repeated use. Typically, sample charging during e-beam exposure or material degradation will broaden or change the secondary electron signal.
In order to reduce edge determination error, SEM calibration has also been done using a sample having a series of equally spaced lines. Such a sample could be a diffraction grating having a plurality of aligned parallel grooves. The SEM is used to measure the pitch of the lines. While this method reduces some of the edge quantification errors associated with other SEM calibration methods, higher accuracy calibration methods are needed for SEMs used for measuring high density semiconductor devices.
Conventional SEM calibration methods and systems do not account for degradation of a calibration standard reference sample over time. For example, where a line width feature on a reference sample has a known width, repeated scanning of the feature by an SEM results in charge buildup. This reduces or hampers the ability of an SEM to obtain accurate measurements of the line width in the future. Contamination on a reference sample feature also prevents or hampers accurate readings. Because conventional SEM calibration methods and systems rely upon accurate SEM readings of a known reference feature dimension, inaccurate SEM readings of a calibration standard reference feature cause errors in measurements of workpiece features performed with the SEM. Inaccurate readings may lead to unnecessary rework of a product lot thereby increasing cost.
The present invention provides a method and system for calibrating a scanning electron microscope, which minimizes or reduces disadvantages associated with conventional methods and systems. In accordance with one aspect of the present invention, there is provided a method for calibrating an SEM using an electrical measurement of a reference sample dimension. A feature, such as for example a conductor line on a reference sample, is measured electrically and measured using a SEM. The electrical and SEM measurements are correlated to determine a critical dimension (CD) for the reference sample feature. A workpiece feature is measured using the SEM, and the reference sample feature CD is correlated with the workpiece feature measurement in order to obtain a workpiece feature CD.
Because electrical measurement is unaffected, or affected differently, by charge buildup and/or other degradation effects on the reference sample, the correlation between the electrical and SEM reference sample measurements can eliminate or reduce the effects of this degradation on system measurements of workpiece features. The electrical reference sample measurement, moreover, may provide trending information relating to degradation of the reference sample over time, as well as an indication of the actual size of a reference feature. In one application of the method, a reference sample is measured electrically, and then optically using a SEM. Thereafter, a workpiece feature is measured, and a workpiece feature CD is obtained using the measurements of the reference sample for correlation.
In accordance with another aspect of the present invention, there is provided a system for calibrating an SEM. The system comprises a reference sample with electrical connections to a probe for measuring a reference sample feature. The system SEM provides an optical measurement of the reference sample feature and correlates the optical and electrical measurements of the reference sample feature to obtain a reference feature CD. The calibration system accounts for degradation in a reference sample associated with repeated usage in a SEM, and further allows trending analysis of the reference sample degradation. Further, the system provides for reduction in the errors associated with initial estimates of the actual reference sample feature size, via an electrical measurement of the feature. Another aspect of the invention provides means for correlating the reference feature CD with a workpiece feature measurement, whereby a workpiece feature CD is obtained.